Environmental standards for particle emissions by coal-fired electrical power plants, petroleum refineries, chemical plants, pulp and paper plants, cement plants, and other particulate-emitting facilities are becoming increasingly more demanding. For example, air quality standards in the United States now require power plants to remove more than 99 percent of the fly ash produced by coal combustion before flue gas may be discharged into the atmosphere. As environmental standards tighten, there is a corresponding need for a more efficient means of particulate removal, particularly in the case of coals having high ash content.
The electrostatic precipitator is a commonly used device for the removal of particles from the exhaust gases produced by the above-noted facilities. There are two primary types of electrostatic precipitators. In the single-stage electrostatic precipitator, the particle-laden gas passes negatively charged corona electrodes which impart a negative charge to the particles. The charged particles then migrate towards positively charged collection plates alternately positioned between the corona electrodes and parallel to the direction of the gas flow. The particles accumulate on the collection plates and are removed by various techniques for disposal.
The two-stage electrostatic precipitator has separate charging and collecting stages. In the charging stage, a series of negatively charged corona electrodes impart a negative charge to the particles. In the collection stage, the negatively charged particles pass through an electric field which causes the charged particles to migrate towards a series of positively charged collection plates. The particles accumulate on the collection plates and are removed by various techniques for disposal. The primary difference between single- and two-stage electrostatic precipitators is that the former combines both the charging stage and the collection stage into a single unit whereas the latter separates the two stages into independent units.
Single- and two-stage electrostatic precipitators are further classified as "hot" and "cold"-side electrostatic precipitators. As used herein, "hot-side electrostatic precipitator" refers to any electrostatic precipitator, whether used by a power plant, petroleum refinery, chemical plant, pulp and paper plant, cement plant, or otherwise, that operates at temperatures above the critical temperature of the particles to be removed, while "cold-side electrostatic precipitator" refers to any electrostatic precipitator operating below the critical temperature of the particles. "Critical temperature" refers to the temperature at which a particle has its highest resistivity to electrical current. By way of example, FIG. 1 illustrates the critical temperature for typical fly ash particles found in utility gas streams. The relationship between particle temperature and particle resistivity exemplified by FIG. 1 exists for other particles treated by electrostatic precipitators, although the precise shape and position of the curve may vary. At temperatures above the critical temperature, particle resistivity is predominantly determined by the chemical composition of the particles and is generally independent of gas characteristics. This relationship between particle resistivity and particle composition makes the particle resistivity inversely proportional to particle temperature. At temperatures below the critical temperature, or in the operating region for cold-side electrostatic precipitators, particle resistivity is predominantly dependent upon the interaction between the particles and the condensable vapors in the gas, such as water and sulfuric acid. This interaction makes resistivity directly proportional to particle temperature.
The efficiency of single-stage electrostatic precipitators is determined to a large extent by the maximum permissible magnitudes of operating voltage and electrical current between the corona electrodes and collection plates. The operating voltage principally determines the strength of the electric field between the corona electrodes and the collection plates and thereby largely establishes the magnitude of the charge imparted to the particles and drawing capability of the collection plates. The corona current, i.e., the flow of ions from the corona electrodes to the collection plates, determines the rate at which particles are charged. Thus, the greater the operating voltage and electrical current, the greater the potential particle removal efficiency of the electrostatic precipitator. Such efficiency is limited, however, by the operating voltage and corona current levels associated with back corona discharge or sparkover occurring in the accumulated particle layer on the collection plates.
Back corona discharge is a phenomena which occurs when the localized electric field generated in the interparticle void spaces in the accumulated particle layer by the ions collecting in the particle layer exceeds the electrical breakdown strength of the gas contained in the interparticle void spaces. As used herein, "localized electric field" refers to the electric field produced by a specified source in a designated area. At higher resistivities of the accumulated particles, the layer becomes more resistant to the flow of negative ions to the positively charged collection plates and the strength of the localized electric field produced in the interparticle void spaces by the charges or ions in the accumulated particle layer correspondingly increases.
When the electric field produced by the accumulated particle layer exceeds the electrical breakdown strength of the accumulated particle layer, i.e., the breakdown strength of the gas in the void spaces between the particles in the accumulated particle layer, electrical energy stored in the accumulated particle layer is discharged, causing an electrical sparkover from the particle layer to the corona electrode and/or reverse ionization. The electrical breakdown strength of the accumulated particle layer is a function of particle size and shape, particle packing density in the accumulated particle layer, and the composition and density of the gas in the interparticle void spaces. In this regard, it is important to understand that the present inventors believe that the onset of back corona discharge is largely unrelated to the thickness of the accumulated particle layer but that the thickness of the accumulated particle layer is directly related to the magnitude of the back corona discharge.
Sparkover caused by back corona discharge limits the operating voltage. Reverse ionization back corona discharge creates a crater in the accumulated particle layer thereby causing a release of positively charged ions into the space between the collection plate and corona electrode. The positively charged ions neutralize the charge on particles produced by negatively charged ions emanating from the corona electrode, resulting in a drain of the operating current and thus a lower operating voltage. As a result, particles receive an inadequate charge to draw them to the collection plates and a greater percentage are discharged into the atmosphere.
The deterioration of efficiencies in hot-side electrostatic precipitators has been studied extensively since efficiency problems began to surface in the late 1970's. The theory most widely recognized in attempting to address the problem is the sodium depletion theory developed by the Southern Research Institute. R. E. Bickelhaupt, Influence of Fly Ash Compositional Factors on Electrical Volume Resistivity, EPA-650/2-74-074 (July 1974). This theory suggests that sodium ions migrate away from the accumulated particle layer nearest the collection plate towards the outer accumulated particle layer boundary. The migration is believed to result in a build-up of a particularly high-resistivity layer in the accumulated particles nearest the collection plates which restricts the flow of negatively charged ions to the plates. Based on the sodium migration theory, a variety of measures have been implemented, including (i) reversing the polarity of the corona electrode and collection plate to reverse the sodium migration; (ii) doping the collection plate with a sodium-based compound; and (iii) increasing the sodium content of the fly ash.
Other methods used in an attempt to decrease the incidence of back corona discharge include: (i) increasing the rapping frequency and intensity or using sonic horns to remove accumulated particles from the collection plates and reduce the thickness of the accumulated particle layer; (ii) energizing the corona electrode in pulses; (iii) using heating devices to adjust the temperature of the input gas and the entire length of the collection plates; (iv) altering the current density in the collection plates along the entire length of the corona electrode; and (v) converting a hot-side electrostatic precipitator to a cold-side electrostatic precipitator. All of the above measures have met with varying degrees of success and none have proven to yield a reliable and practical solution to the efficiency problems plaguing hot-side electrostatic precipitators.
By way of example, increasing the frequency of particle removal by rapping the collection plates has been found to actually increase reentrainment of the particles into the gas stream, which decreases electrostatic precipitator efficiency. Many of the dislodged particles fall into the hopper but some particles are reintroduced into the gas stream. Field studies have shown that as much as 80 percent of the particulate emissions from electrostatic precipitators occurs as a result of particle removal from the collection plates. There have also been occasions where high rapping frequencies distorted the support hangers for the collection plates, especially when coupled with the additional weight caused by accumulations of particles on the collection plates. Distortions in the support hangers produce a misalignment of the collection plates leading to subsequent electrode failure.
One proposed apparatus utilizing the approach of increasing the temperature of the input gas and/or the entire electrostatic precipitator, including the corona electrodes and collection plates, is disclosed by U.S. Pat. No. 4,431,434. Specifically, an electrostatic precipitator is disclosed which has portions of the corona electrodes and collection plates constructed of hollow tubes through which a temperature control fluid is passed to control particle temperature, in an attempt to maintain particle resistivity in a range in which back corona discharge will not be as likely to occur. Such electrostatic precipitators are relatively expensive to construct, requiring tubular configurations, heating units and pumps, and are also expensive to operate. Such an 15 approach to addressing the problem also does not provide a practical means to modify existing electrostatic precipitators to reduce the incidence of back corona discharge.
An electrostatic precipitator incorporating the approach of altering the current density in the collection plates along the entire length of the corona electrode is disclosed in U.S. Pat. No. 4,518,401. In particular, an electrostatic precipitator is described having corona electrodes having a diameter from top to bottom that is approximately three times larger than the diameter of corona electrodes used in typical conventional electrostatic precipitators. This approach substantially reduces efficiencies as a result of the lower rate of particle charging caused by a decreased current density along the entire length of the corona electrode. Further, implementation of this approach for existing electrostatic precipitators may be impractical since all existing corona electrodes would need to be replaced by larger diameter electrodes.
The retrofit approach of converting hot-side electrostatic precipitators to cold-side electrostatic precipitators with the addition of flue gas conditioning, conversion to a cold-side fabric filter baghouse, and enlargement of the existing hot-side electrostatic precipitator, is very expensive. The conversion involves extensive modification to the existing duct work and relocation of the air preheater. It is estimated that such conversions currently cost from about $15 million to $35 million. Worse yet, the conversion does not guarantee that emission limits will be met after the conversion or that the incidence of back corona discharge will be eliminated.
A fundamental problem with each of the foregoing attempts to address the back corona discharge problem in electrostatic precipitators is the focus by industry on altering the structure or operation of the entire electrostatic precipitator instead of focusing on those isolated sections of the electrostatic precipitator in which back corona discharge occurs most frequently.
It is an object of the present invention to reduce the degradation in hot-side, single-stage electrostatic precipitator performance attributed to back corona discharge by developing not only an improved design for hot-side, single-stage electrostatic precipitators but also a practical alternative for modifying existing hot-side, single-stage electrostatic precipitators to substantially reduce back corona discharge.